The disturbed man who was shot to death by police at a Brooklyn synagogue had planned to play soccer with his sons on Monday — but a bi-polar episode sent him on a violent rampage instead, his brother told The Post.

Calvin Peters, 49 — who stabbed a 22-year-old man at Chad-Lubavitch headquarters in Crown Heights on early Tuesday — flipped out because he had stopped taking his medication, said the brother, Glen Peters, 53.

Calvin Peters

“I spoke to him on the phone [Monday]… I knew something was wrong right away because he was acting very strange. He told me he was hanging out in Brooklyn. That was definitely wrong because he never goes to Brooklyn,” Peters said.

Stabbing victim Levi Rosenblat, who is currently in stable condition at Kings County Hospital.

“He was afraid and paranoid and ill. You could tell he was mental…He just needed to be back on his medication. He needed to be restrained and go to a hospital so he could go back to normal,” he said.

“His life didn’t need to end this way…My brother is gone and I feel so lost. My entire family is in such a state of grief,” he said. Calvin Peters — who had racked up a lengthy criminal record during his decades-long struggle with mental illness — left his home in Valley Stream, LI, for a dental appointment at around 11 a.m. on Monday, according to police sources and family members.

But he never made it to the dentist. And his brother didn’t hear from him until around 3 p.m., he said.

“At that time, he was supposed to be playing soccer with his two sons. He told me his wife, Gayle, was going to take them back and forth to practice,” Peters said.

“I told him to just go home. He wasn’t having it. He said he wanted to stay for some reason but he wouldn’t say what. I was very concerned and called my father who said he would try talking to him,” Peters said.

“That was the last I heard of him,” he said.

Two hours later, at 5:15 p.m. on Monday, Calvin Peters visited the synagogue, where he acted “highly suspicious,” according to security guard Shimon Liberow.

Liberow kicked him out — but came back at around 12: 45 a.m. on Tuesday, acting even more bizarre.

A blood-stained 9-inch knife was recovered at the scene of the stabbing.Reuters

Onlookers thought he might be high on drugs as he mumbled nonsense about the Bible and converting to Judaism before flipping out stabbing Shimon Liberow, 22, police sources said. Peters had 19 prior arrests for charges including larceny, possession of controlled substance, using a credit card that wasn’t his, contempt of court and burglary.

But he wasn’t always a bad guy, his brother said. He was diagnosed with mental illness in 1982 after his life began to spiral out of control.

A member of the Lubavitch community leaves the headquarters Tuesday morning.AP

“[He had] been sick for a long time and he couldn’t hold down a job…He was well educated, it’s just that he became ill…He was never violent. Every time he was arrested it was for some drugs or just acting crazy,” his brother said.

The world headquarters of Chabad-Lubavitch at 770 Eastern Parkway.AP

Witnesses said they heard Peters say he wanted to “kill Jews” — but he had never uttered an antisemitic word prior to that, Glen Peters said.

“I don’t recall him ever saying he hates Jews or even attacking anyone.”

His heartbroken family blasted the officer who killed him on Tuesday afternoon, saying there had to be another option.

“We want to know why the officer had to kill him. There was a moment where Calvin put down the knife and moved away from it. The officer holstered his weapon and turned around. He hesitated!” Peters said.

“He should have just rushed Calvin while he was unarmed and cuffed him or taken him down. Or even then, he could have just tasered him. The officer was so close to my brother at that point, he could have done anything he wanted. He could have shot him in the leg 10 times if he needed to. Why did he have to aim for the heart?” he said.

Distraught, he added, “I know what he did was wrong. It was horrible. But there had to be a better way — aside from taking his life.”